Fatal Flaws; 5 big mistakes people make when planning a major renovation

Renovating an old Inner West terrace sounds simple—open up the back, shuffle a few rooms, pick some fresh colours. Yet long before you worry about paint or whether the lounge matches the rug, you can stumble into deep planning errors that smash budgets and stretch timelines. I call these errors the fatal flaws. They lurk behind brickwork, under floors and inside ceilings, and they will punish anyone who ignores them. Let’s walk through the five most common traps and see how to avoid each one.

1. Assuming the House Is Square, Level and Plumb

Many Inner Sydney homes were built a century ago and have shifted in the years since. Walls lean, floors sag and nothing is perfectly plumb or level. If you draw every new room as a neat rectangle and pre-order custom joinery or stone, those pieces won’t fit once demolition reveals the truth. Trades end up packing and shaving everything, adding time and cost. Get a thorough site measure early and leave tolerance wherever new work meets old—accept that your house is charmingly crooked and design around it.

2. Forgetting What Sits Under a Wall

Plans often show walls as two tidy lines but skip the footings below. When you keep an old wall or add a new structural one - or dig down below the footing levels - you may need concrete underpins up to 500 mm wider than the wall itself. In a narrow hallway or tight kitchen, that hidden bulk steals precious space. Make sure footings and underpins appear on drawings from day one— with a realistic width and depth—so the floor plan you fall in love with is the one you actually build.

3. Missing a Drainable Cavity Below Ground

Digging down to gain head-height or add a wine store is popular, but any wall that now retains soil is now presumed to be a “wet wall”. A standard cavity wall won’t cut it. You need a drained cavity that collects water and sends it to an outlet, plus a small step down from the internal floor so moisture can’t creep back in. Skip this and the wall has to thicken mid-build, chewing up space and halting work while everyone finds a fix.

4. Ignoring Gravity in the Sewer Line

Sewer pipes only flow downhill. Lower a floor below the main sewer, or run a pipe dead-flat in a ceiling, and you risk non compliance with plumbing code. Even shifting a bathroom far across the plan can leave no fall by the time the pipe reaches the street. Check levels in week one and give every line the slope it needs—long before you pour the slab.

5. Squeezing Structure and Services into No Space

Modern living comes with big pieces of equipment: air-con condensers, gas hot-water units that must sit outside clear of windows, and electric tanks that need elbow-room for servicing. Above ceilings, fat steel beams for wide openings share space with sewer and storm-water pipes. Trim that zone too far, and you lose ceiling height or lock pipes into places they can’t be reached later. Map every beam, pipe and duct early and allow honest depths instead of hoping they’ll “just squeeze in.”

Renovation should feel exciting, not like dodging one crisis after another. Catch these five fatal flaws while you’re still sketching ideas on a café napkin, and your build will stay on budget, on schedule and free of nasty surprises once the walls come down.

If you’re planning a renovation and want to steer clear of costly missteps, our Design Consultation is a good place to start. We’ll help you spot the risks early—before they become expensive problems.